How to use the less/tree.Expression function in less

eval: function (env) {
        if (this.value.length > 1) {
            return new(tree.Expression)(this.value.map(function (e) {
                return e.eval(env);
            }));
        } else {
            return this.value[0].eval(env);
        }
    },
    toCSS: function () {

eval: function (env) {
        if (this.value.length > 1) {
            return new(tree.Expression)(this.value.map(function (e) {
                return e.eval(env);
            }));
        } else {
            return this.value[0].eval(env);
        }
    },
    toCSS: function () {

expression: function () {
                var e, delim, entities = [], d;

                while (e = $(this.addition) || $(this.entity)) {
                    entities.push(e);
                }
                if (entities.length > 0) {
                    return new(tree.Expression)(entities);
                }
            },
            property: function () {

eval: function (env) {
        if (this.value.length > 1) {
            return new(tree.Expression)(this.value.map(function (e) {
                return e.eval(env);
            }));
        } else {
            return this.value[0].eval(env);
        }
    },
    toCSS: function () {

font: function () {
                var value = [], expression = [], weight, shorthand, font, e;

                while (e = $(this.shorthand) || $(this.entity)) {
                    expression.push(e);
                }
                value.push(new(tree.Expression)(expression));

                if ($(',')) {
                    while (e = $(this.expression)) {
                        value.push(e);
                        if (! $(',')) { break }
                    }
                }
                return new(tree.Value)(value, $(this.important));
            },

expression: function () {
                var e, delim, entities = [], d;

                while (e = $(this.addition) || $(this.entity)) {
                    entities.push(e);
                }
                if (entities.length > 0) {
                    return new(tree.Expression)(entities);
                }
            },
            property: function () {

expression: function () {
                var e, delim, entities = [], d;

                while (e = $(this.addition) || $(this.entity)) {
                    entities.push(e);
                }
                if (entities.length > 0) {
                    return new(tree.Expression)(entities);
                }
            },
            property: function () {

font: function () {
                var value = [], expression = [], weight, shorthand, font, e;

                while (e = $(this.shorthand) || $(this.entity)) {
                    expression.push(e);
                }
                value.push(new(tree.Expression)(expression));

                if ($(',')) {
                    while (e = $(this.expression)) {
                        value.push(e);
                        if (! $(',')) { break }
                    }
                }
                return new(tree.Value)(value, $(this.important));
            },

font: function () {
                var value = [], expression = [], weight, shorthand, font, e;

                while (e = $(this.shorthand) || $(this.entity)) {
                    expression.push(e);
                }
                value.push(new(tree.Expression)(expression));

                if ($(',')) {
                    while (e = $(this.expression)) {
                        value.push(e);
                        if (! $(',')) { break }
                    }
                }
                return new(tree.Value)(value, $(this.important));
            },